I. Technical Field
The present invention relates to an optical recording/reproduction method and an optical recording/reproduction device for optically recording or reproducing signals using near-field, a program, and an optical recording medium.
II. Description of the Related Art
Recording or reproducing on an optical recording medium, such as a DVD, is performed using a spot of light that has converged through an objective lens. The spot size of light will become smaller as the wavelength is shorter, or as the numerical aperture (NA) of the objective lens is larger. The smaller light spot enables recording and reproducing at higher densities. Many efforts have conventionally been made to achieve high density recording and reproduction. Among these efforts, an optical system using a solid immersion lens (SIL) has recently received attention as a technique that significantly increases the NA of an objective lens. The use of the SIL enables the refractive index of light, which is 1 in the atmosphere, to increase at a light incident side in accordance with the composition of the SIL. This can significantly increase the NA of the objective lens.
To efficiently transfer light from the SIL to an optical recording medium using near-field light, the distance between the SIL and the optical recording medium (hereafter referred to as the “gap”) needs to be as small as a fraction of the wavelength of the light. The need for such a small gap will cause new problems that are not seen in conventional optical systems.
Before such problems are discussed, an example of a recording/reproduction device using an SIL will now be described with reference to FIGS. 2A and 2B.
As shown in FIG. 2A, light emitted from a light source 201 is collimated through a collimator 202, and the collimated beam then passes through a polarizing beam splitter (PBS) 203 and a quarter-wavelength plate 204. Subsequently, the light beam passes through an objective lens including a convex lens 205 and an SIL 206, and converges on the bottom surface of the SIL. After converging on the bottom surface of the SIL, the light beam then travels through near-field light and reaches a signal layer of an optical recording medium 207. The light beam is reflected and passes through the objective lens and the quarter-wavelength plate. The light beam is then reflected on the PBS 203, and converges on a photo detector (PD) 209 through a convergence lens 208. As shown in FIG. 2B, the PD 209 is divided in four areas 209A, 209B, 209C, and 209D. The light converging through the convergence lens 208 is controlled to form a spot 210 at the center of the entire area of the PD 209 consisting of the four areas. Signals transmitted from the four areas are used for information reproduction, focusing, tilting, and various other purposes. The objective lens is adjustable horizontally, vertically, and in the direction of tilt using an actuator 211, a tilt adjustment unit 212, and the like. For ease of explanation, FIGS. 2A and 2B only show a simplified basic structure of the optical recording/reproduction device. A typical recording/reproduction device can often include different light sources and different PDs for recording and reproduction and for servo control. Depending on purposes, the recording/reproduction device can also use PDs that are divided in two areas or PDs that are not divided in a plurality of areas.
The problems associated with the SIL optical system will now be described.
A first problem is that the gap of as small as substantially a ten-thousandth part of the conventional gap between the SIL and the optical recording medium is extremely difficult to maintain using conventional techniques. A gap servo, which is described for example in Domestic Re-publication of International Publication No. 03/021583, has been proposed as an effective technique to solve this problem. With the gap servo technique, the amount of reflected light from the total reflection area formed on the bottom surface of the SIL, which varies depending on the gap, is detected using a PD 209, a signal determined according to the detected amount of reflected light is then input into a gap servo circuit, and an actuator 211 is driven using the gap servo circuit in a manner that the gap will be constant. As shown in FIG. 3, the amount of reflected light from the total reflection area is constant when the gap is sufficiently large. In a near-field area, the gap is substantially half the wavelength or less. In that area, the amount of reflected light tends to decrease as the gap becomes smaller. Based on this tendency. the gap can be estimated using the reflected light amount.
Also, because the SIL optical system provides such an extremely small gap between the SIL and the optical recording medium, even slight tilting of the surface of the optical recording medium and the bottom surface of the SIL relative to each other can cause the two surfaces to contact each other. A tilt servo, which uses reflected light from the total reflection area of the bottom surface of the SIL like the gap servo, has been proposed as a technique to solve this problem. With the tilt servo technique, the asymmetry of reflected light occurring when the gap between the SIL and the optical recording medium becomes uneven due to tilting is detected using a PD 209, and a tilt adjustment unit is driven using a tilt servo circuit in a manner that the tilting will be eliminated accordingly. The tilt servo may be performed using a light source different from a light source used for recording and reproduction, or may also be performed using the same light source as that for recording and reproduction. The tilt servo technique that uses the same light source both for the tilt servo and for recording and reproduction is strongly preferred because this technique requires a lower manufacturing cost and enables easier adjustment of the optical system. However, the tilt servo technique that uses the same light source has yet to be commercialized due to the problems described below. Examples of the technique that uses the same light source both for the tilt servo and for recording and reproduction are described in Japanese Unexamined Patent Publication No. 2005-259329 and Japanese Unexamined Patent Publication No. 2006-4596.
Patent Citation 1: Domestic Re-publication of International Publication No. 03/021583
Patent Citation 2: Japanese Unexamined Patent Publication No. 2005-259329
Patent Citation 3: Japanese Unexamined Patent Publication No. 2006-4596